JP6092885B2 - Negative electrode active material for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery using the negative electrode active material - Google Patents

Description

  The present invention relates to a negative electrode active material for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the negative electrode active material.

Since the silicon oxide represented by SiO _X has a high specific capacity and a volume expansion coefficient when absorbing lithium during charging is smaller than that of Si, it has been studied to mix with graphite and use it as a negative electrode active material. (See Patent Document 1).
However, the nonaqueous electrolyte secondary battery using the silicon oxide represented by SiO _X as the negative electrode active material has significantly higher initial charge / discharge efficiency and capacity at the beginning of the cycle than when only graphite is used as the negative electrode active material. There is a problem of lowering.
In order to improve the initial charge and discharge efficiency, composite particles having a structure in which silicon oxide is dispersed in a carbon active material and silicon and a lithium silicate phase are included in the silicon oxide have been proposed (Patent Document 2). reference).

JP2011-233245A JP 2007-59213 A

  However, in the proposal described in Patent Document 2, since the silicon oxide dispersed in the carbon active material has a structure in which the silicon oxide is scattered in the carbon matrix, the carbon matrix inhibits lithium diffusion during charging and discharging. To do. For this reason, lithium may not reach the silicon oxide sufficiently, and the actual battery capacity is significantly smaller than the theoretical capacity, and the initial charge / discharge efficiency is reduced.

The negative electrode active material particles for a non-aqueous electrolyte secondary battery of the present invention are provided with carbon only on the surface thereof. The particles include SiO _X (0.8 ≦ X ≦ 1.2) particles including a Li silicate phase . The surface of the SiO _X particles is covered with carbon by 50% or more and 100% or less.

According to the embodiment of the present invention, in the nonaqueous electrolyte secondary battery using SiO 2 _X as the negative electrode active material, the initial charge / discharge efficiency and the cycle characteristics are dramatically improved.

It is a graph showing the XRD measurement result of SiO _X in cell A1, Z.

  In this specification, “substantially **” is intended to include not only exactly the same, but also those that are recognized as substantially the same, with “substantially equivalent” as an example.

The negative electrode active material of the present invention is a particle composed of SiO _X (0.8 ≦ X ≦ 1.2) containing a lithium silicate phase inside, and the surface of the particle composed of SiO _X is 50% or more and 100% of carbon. % Or less is covered.
In the battery using the negative electrode active material having the above structure, the initial charge / discharge efficiency and the cycle characteristics can be improved. The reason is shown below.

SiO _X is a fine mixture of Si and SiO _2, and the initial charge reaction when used as a negative electrode active material can be generally expressed by the following formula (1).
4SiO (2Si + 2SiO ₂ ) + 16Li ⁺ + 16e ⁻ → 3Li ₄ Si + Li ₄ SiO ₄ (1)
As in the above formula (1), Li ₄ SiO ₄ is generated during the initial charge, and this Li ₄ SiO ₄ is an irreversible reactant. Therefore, not all Si in SiO _X reacts reversibly, and the theoretical efficiency is lowered. Specifically, when Li ₄ SiO ₄ is generated as an irreversible reactant as in the above formula (1), four of the 16 lithium ions are irreversible, so the theoretical efficiency is 75%. Become.

Therefore, as described above, SiO _{X in} which a lithium silicate phase such as Li ₄ SiO ₄ is formed is used as SiO _X at the time of battery fabrication (before the first charge). With such a configuration, the amount of lithium taken away by the irreversible reactant during the first charge is reduced, so that the first charge / discharge efficiency can be drastically improved. In addition, the volume of the SiO _X particles is increased by forming a lithium silicate phase. Therefore, in the case of using SiO _X as a negative electrode active material, SiO _X having lithium silicate phase, the expansion during charge and discharge than SiO _X having no lithium silicate phase displacement during shrinkage is small. Therefore, if SiO _X having a lithium silicate phase is used, peeling in the negative electrode mixture layer and peeling between the negative electrode mixture layer and the negative electrode current collector can be suppressed, and thus cycle characteristics are improved. In addition, since there is no carbon matrix around SiO _X , lithium diffusion is performed smoothly. Therefore, the actual battery capacity is increased.

The lithium silicate phase may be composed of not only Li ₄ SiO ₄ but also Li ₂ SiO ₃ or the like, but in any case, it is electrochemically inactive. Further, the lithium silicate phase is not formed electrochemically, but is formed by a chemical reaction. For example, it can be formed by the following method.

In order to form a lithium silicate phase in SiO _X , for example, a lithium compound such as LiOH, Li ₂ CO ₃ , LiF, or LiCl can be mixed with SiO _X and heat-treated at a high temperature. In this case, the reaction formula when LiOH is used as the lithium compound is shown in the following formula (2). As is clear from the formula (2), it can be seen that SiO ₂ existing in SiO _X reacts with LiOH to produce Li ₄ SiO ₄ .
SiO ₂ + 4LiOH → Li ₄ SiO ₄ + 2H ₂ O (2)
The lithium silicate phase is a compound of Li, Si, and O. In addition to Li ₄ SiO ₄ , there are Li ₂ SiO ₃ and Li ₂ Si ₂ O ₅ , and the product depends on the amount of lithium compound added and the processing method. May be different.

The ratio of the lithium silicate phase to the total amount of SiO _X (0.8 ≦ X ≦ 1.2) particles is preferably 0.5 mol% or more and 25 mol% or less. When the proportion of the lithium silicate phase is less than 0.5 mol%, the effect of improving the initial charge / discharge efficiency is small. On the other hand, when the proportion of the lithium silicate phase exceeds 25 mol%, the amount of Si that undergoes reversible reaction decreases, and the charge / discharge capacity decreases.

The surface of SiO _X used in the present invention is 50% or more and 100% or less, preferably 100%, of carbon. When SiO _X surface covered 50% to 100% carbon, when to form a lithium silicate phase in SiO _X, it is possible to suppress the lithium compound and SiO _X is direct contact, the interior of the SiO _X particles This is because it is possible to uniformly react lithium with SiO _X. In the present invention, the SiO _X surface is covered with carbon means that the surface of the SiO _X particle is covered with a carbon film having a thickness of at least 1 nm when the particle cross section is observed by SEM. . In the present invention, the SiO _X surface is 100% coated with carbon. When the particle cross section is observed by SEM, almost 100% of the SiO _X particle surface is covered with a carbon film having a thickness of at least 1 nm. That's what it means. When coating with carbon, to enhance the reaction uniformity of SiO _X, it is preferable to uniformly coat the surface of the SiO _X. The thickness of the carbon coating is preferably 1 nm or more and 200 nm or less. If it is less than 1 nm, the conductivity is low and it is difficult to coat uniformly. On the other hand, if it exceeds 200 nm, the carbon coating inhibits lithium diffusion, so that lithium does not reach SiO _X sufficiently and the capacity is greatly reduced. Further, in the case of carbon coating, the ratio of carbon to SiO _X is desirably 10% by mass or less.

The average primary particle diameter of SiO _X used in the present invention is preferably 1 μm or more and 15 μm or less. When the average primary particle diameter of SiO _X is less than 1 μm, the particle surface area becomes too large, the amount of reaction with the electrolytic solution increases, and the capacity may decrease. Further, the amount of expansion and contraction of SiO _X is small, and the influence on the negative electrode mixture layer is small. Therefore, even if a lithium silicate phase is not formed in advance in SiO _X , separation between the negative electrode mixture layer and the negative electrode current collector hardly occurs and the cycle characteristics do not deteriorate so much. On the other hand, when the average primary particle diameter of SiO _X exceeds 15 μm, lithium may not diffuse into the inside of SiO _X during the formation of the lithium silicate phase, and the lithium silicate phase may be formed only on the SiO _X surface. . Since the lithium silicate phase is insulative, when such a structure is used, lithium diffusion is hindered, and lithium cannot be diffused to the vicinity of the center of SiO during charge / discharge, which may result in a decrease in capacity and load characteristics. Therefore, the average primary particle diameter of SiO _X is preferably 1 μm or more and 15 μm or less, and particularly preferably 4 μm or more and 10 μm or less.
In addition, the average primary particle diameter (D ₅₀ ) of SiO _X is the cumulative 50 volume% diameter in the particle size distribution measured by the laser diffraction scattering method.

SiO _X used in the present invention may be used alone as a negative electrode active material, or may be used by mixing with a carbon-based active material such as graphite or hard carbon. Since the specific capacity of SiO _X is higher than that of the carbon-based active material, the capacity can be increased as the addition amount increases. However, SiO _X has a larger expansion / contraction rate at the time of charge / discharge than the carbon-based active material, and if the ratio is too large, peeling at the interface between the negative electrode mixture layer and the negative electrode current collector, or the negative electrode active material Since the conductive contact between the particles is reduced, the cycle characteristics may be significantly reduced. Therefore, when SiO _X and a carbon-based active material are mixed and used, the ratio of SiO _{X to} the total amount of the negative electrode active material is preferably 20% by mass or less. On the other hand, if the proportion of SiO _X is too small, the merit of increasing the capacity by adding SiO _X is reduced, so the proportion of SiO _X with respect to the total amount of the negative electrode active material is preferably 1% by mass or more.

The positive electrode and the non-aqueous electrolyte can be used without any particular limitation as long as they are used for a non-aqueous electrolyte secondary battery.
Examples of the positive electrode active material include lithium complex oxide containing lithium cobaltate, nickel or manganese, olivine type lithium phosphate represented by lithium iron phosphate (LiFePO ₄ ), and the like. Examples of the lithium composite oxide containing nickel or manganese include lithium composite oxides such as Ni—Co—Mn, Ni—Mn—Al, and Ni—Co—Al. These positive electrode active materials may be used alone or in combination.

When the positive electrode active material contains an oxide containing lithium and a metal element M, and the metal element M contains at least one selected from the group containing cobalt and nickel, the amount of lithium contained in the positive electrode and the negative electrode and the sum x, the ratio x / _{M C} to the amount _{M C} of the metal element M contained in the oxides of the above, for example, preferably greater than 1.01, more preferably greater than 1.03.
If the ratio x / M _C is within the above range, so that the proportion of lithium ions supplied to the battery is quite large. That is, it is advantageous in terms of compensation for irreversible capacity.

The ratio x / M _C, for example, when the anode active material is a mixture of a SiO _X and the carbon-based active material containing lithium silicate phase therein, the proportion of SiO _X such with respect to the total amount of the anode active material, fluctuate.
The ratio x / M _C is the amount M _C of the metal element M contained in the lithium content x and the positive electrode active material contained in the positive electrode and the negative electrode, were quantified, respectively, dividing the amount of x in an amount M _C of the metal element M This can be calculated.

The amount M _C of lithium content x and the metal element M can be quantified as follows.
First, the battery is completely discharged and then decomposed to remove the nonaqueous electrolyte, and the inside of the battery is washed with a solvent such as dimethyl carbonate. Next, the positive electrode and the negative electrode are respectively collected by a predetermined mass, and the amount of lithium (molar amount) x is determined by quantifying the amount of lithium contained in the positive electrode and the negative electrode by ICP analysis. Also, as in the case of the amount of lithium in the positive electrode, the amount of metal element M contained in the positive electrode (molar amount) determined by the M _C ICP analysis.

The solvent and solute of the nonaqueous electrolyte solution are not particularly limited as long as they can be used for the nonaqueous electrolyte secondary battery.
Solutes of the non-aqueous electrolyte include LiBF ₄ , LiPF ₆ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiPF _6-x (C _n F _{2n + 1} ) _x [wherein 1 <x <6, n = 1 or 2], or a lithium salt having an oxalato complex as an anion can also be used. As a lithium salt having this oxalato complex as an anion, in addition to LiBOB [lithium-bisoxalate borate], a lithium salt having an anion in which C ₂ O ₄ ²⁻ is coordinated to the central atom, for example, Li [M (C ₂ O ₄ ) _x R _y ] (wherein M is a transition metal, an element selected from groups IIIb, IVb, and Vb of the periodic table, R is selected from a halogen, an alkyl group, and a halogen-substituted alkyl group) Group, x is a positive integer, and y is 0 or a positive integer). Specifically, there are Li [B (C ₂ O ₄ ) F ₂ ], Li [P (C ₂ O ₄ ) F ₄ ], Li [P (C ₂ O ₄ ) ₂ F ₂ ], and the like. However, it is most preferable to use LiBOB in order to form a stable film on the surface of the negative electrode even in a high temperature environment.

  In addition, the said solute may be used not only independently but in mixture of 2 or more types. The concentration of the solute is not particularly limited, but is preferably 0.8 to 1.8 mol per liter of the electrolyte. Furthermore, in applications that require discharging with a large current, the concentration of the solute is desirably 1.0 to 1.6 mol per liter of the electrolyte.

  On the other hand, as the solvent of the non-aqueous electrolyte, carbonate solvents such as ethylene carbonate, propylene carbonate, γ-butyl lactone, diethylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and a part of hydrogen in these solvents are F. Substituted carbonate solvents are preferably used. As the solvent, it is preferable to use a combination of a cyclic carbonate and a chain carbonate.

The difference from the invention described in Patent Document 2 is as follows.
(1) As described above, also in the present invention, the surface of SiO _X is coated with carbon. Accordingly, not only the invention described in Patent Document 2 but also the present invention includes carbon in the SiO _X particles. However, in the invention described in Patent Document 2, carbon exists up to the inside of the particle, whereas in the present invention, carbon exists only on the surface of the particle. In relation to this, the proportion of carbon in the particles is about 10% by mass or less and extremely low in the present invention, whereas in the invention described in Patent Document 2, it is about 50% by mass or more. It is extremely numerous.

(2) In the invention described in Patent Document 2, heat treatment is performed in the presence of SiO powder, carbon powder, and a lithium compound. Therefore, lithium of the lithium compound is taken into carbon as well as SiO. On the other hand, in this invention, after heat-processing in presence of SiO powder and a lithium compound, it mixes with carbon powder. Therefore, lithium of the lithium compound is taken into only SiO (not taken into carbon).

(3) as in the embodiment described in Patent Document 2, when the structure SiO _X is interspersed within a carbon matrix, the particle size of the SiO _X is small and, in SiO _X is covered with a carbon matrix that can relieve stress Yes. Accordingly, the influence (exfoliation between the negative electrode current collector and the negative electrode mixture layer) on the negative electrode mixture layer due to the expansion and contraction of the negative electrode active material during charge / discharge is extremely small. For this reason, in the invention described in Patent Document 2, the effect of improving battery characteristics by relaxing expansion and contraction of the negative electrode active material is only slightly exhibited.
In contrast, as in the present invention, when used as a mixture with particles consisting of SiO _X containing internal lithium silicate phase (single particles of SiO _X) and graphite, in order to suppress a side reaction with the electrolyte solution In addition, it is necessary to increase the particle size of SiO _X to some extent, and there is no matrix around SiO _X that can relieve stress. Therefore, the influence exerted on the negative electrode mixture layer by the expansion and contraction of the negative electrode active material is extremely large. For this reason, in this invention, the effect of improving a battery characteristic is remarkably exhibited by relieving the expansion | swelling and shrinkage | contraction of a negative electrode active material.

Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the present invention is not limited to the following examples, and can be implemented with appropriate modifications without departing from the scope of the present invention. It is a thing.
<First embodiment>

Example 1
(Production of negative electrode)
SiO _X (X = 0.93, average primary particle size: 5.0 μm) whose surface was coated with carbon was prepared. The coating was performed using a CVD method, the ratio of carbon to SiO _X was 10 mass%, and the carbon coverage on the SiO _X surface was 100%. 1 mol of SiO _X and 0.2 mol of LiOH were mixed in a powder state (the ratio of LiOH to SiO _X is 20 mol%), and LiOH was adhered to the surface of SiO _X. Next, heat treatment was performed in an Ar atmosphere at 800 ° C. for 10 hours to produce SiO _X in which a lithium silicate phase was formed. When SiO _X after this heat treatment was analyzed by XRD (the radiation source was CuKα), as shown in FIG. 1, the peaks of Li ₄ SiO ₄ and Li ₂ SiO ₃ which are lithium silicates were confirmed. Further, the number of moles of lithium silicate phase to the total number of moles of SiO _X (hereinafter, the ratio of the lithium silicate phase in the SiO _X, may be referred to as) was 5 mol%.

The carbon coverage on the SiO _X surface was confirmed by the following method. Using an ion milling device (ex. IM4000) manufactured by Hitachi High-Tech, the cross section of the negative electrode active material particles was exposed, and the particle cross section was confirmed by SEM and a backscattered electron image. The interface between the carbon coating layer in the particle cross section and SiO _X was specified from the reflected electron image. Then, the ratio of the carbon film having a film thickness of 1 nm or more on the surface of each SiO _X particle was calculated from the ratio of the sum of the interface lengths of the carbon film having a film thickness of 1 nm or more and SiO _X to the outer peripheral length of SiO _X in the particle cross section. . The average value of the ratio of the carbon coating of 30 SiO _X particles was defined as the carbon coverage.

SiO _X in which the lithium silicate phase is formed and PAN (polyacrylonitrile) as a binder are mixed at a mass ratio of 95: 5, and NMP (N-methyl-2-pyrrolidone as a dilution solvent) is further mixed. ) Was added. This was stirred using a mixer (Primics, Robomix) to prepare a negative electrode mixture slurry.
The negative electrode mixture slurry was applied on one surface of a copper foil such that the mass per lm ^{2 of the} negative electrode mixture layer was 25 g / m ² . Next, this was dried at 105 ° C. in the atmosphere and rolled to prepare a negative electrode. The filling density of the negative electrode mixture layer was 1.50 g / ml.

(Preparation of non-aqueous electrolyte)
To a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7, lithium hexafluorophosphate (LiPF ₆ ) was added at 1.0 mol / liter. This was added to prepare a non-aqueous electrolyte.

[Assembling the battery]
In an inert atmosphere, an electrode body was produced using the above negative electrode with a Ni tab attached to the outer periphery, a lithium metal foil, and a polyethylene separator disposed between the negative electrode and the lithium metal foil. This electrode body was put into a battery casing made of an aluminum laminate, and a non-aqueous electrolyte was injected into the battery casing, and then the battery casing was sealed to produce a battery. The battery thus produced is hereinafter referred to as battery A1.

(Example 2)
When mixing and heat-treating the lithium source and SiO _X , Li ₂ CO ₃ was used instead of LiOH as the lithium source (the ratio of Li ₂ CO ₃ to SiO _X was 10 mol%), A battery was fabricated in the same manner as in Example 1 of the first example. Incidentally, the SiO _X after heat treatment, was analyzed by XRD, the peak of the _Li 4 SiO ₄ and _Li 2 SiO ₃ is a lithium silicate was confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 5 mol%. The battery thus produced is hereinafter referred to as battery A2.

Example 4
When mixing and heat-treating the lithium source and SiO _X , LiCl was used instead of LiOH as the lithium source (the ratio of LiCl to SiO _X was 20 mol%). A battery was produced in the same manner as in Example 1. Incidentally, the SiO _X after heat treatment, was analyzed by XRD, the peak of the _Li 4 SiO ₄ and _Li 2 SiO ₃ is a lithium silicate was confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 5 mol%. The battery thus produced is hereinafter referred to as battery A3.

Example 4
When the heat treatment was performed by mixing the lithium source and SiO _X , LiF was used instead of LiOH as the lithium source (the ratio of LiF to SiO _X was 20 mol%). A battery was produced in the same manner as in Example 1. Incidentally, the SiO _X after heat treatment, was analyzed by XRD, the peak of the _Li 4 SiO ₄ and _Li 2 SiO ₃ is a lithium silicate was confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 5 mol%. The battery thus produced is hereinafter referred to as battery A4.

(Comparative example)
Example of the first example except that LiOH and SiO _X were not mixed and heat treatment was not performed (that is, untreated SiO _X was used as SiO _X as the negative electrode active material). A battery was produced in the same manner as in Example 1. When this SiO _X was analyzed by XRD, a lithium silicate phase was not confirmed as shown in FIG. The battery thus produced is hereinafter referred to as battery Z.

(Experiment)
The batteries A1 to A4, Z were charged / discharged under the following conditions, and the initial charge / discharge efficiency represented by the following formula (3) and the capacity retention rate at the 10th cycle represented by the following formula (4) were examined. The results are shown in Table 1.
(Charging / discharging conditions)
Constant current charging was performed until the voltage became 0 V with a current of 0.2 It (4 mA), and then constant current charging was performed until the voltage became 0 V with a current of 0.05 It (1 mA). Next, after resting for 10 minutes, constant current discharge was performed until the voltage became 1.0 V at a current of 0.2 It (4 mA).

[Calculation formula for initial charge / discharge efficiency]
Initial charge / discharge efficiency (%) = (discharge capacity at the first cycle / charge capacity at the first cycle) × 100 (3)
[Calculation formula of capacity maintenance ratio at 10th cycle]
Capacity maintenance ratio (%) at 10th cycle = (discharge capacity at 10th cycle / discharge capacity at 1st cycle) × 100 (4)

Batteries A1 to A4 using SiO _X having a lithium silicate phase inside have improved initial charge / discharge efficiency and cycle characteristics as compared to battery Z using SiO _X having no lithium silicate phase inside. I understand. This is because if SiO _X before charge / discharge has a lithium silicate phase in advance, the amount of lithium taken away by Li ₄ SiO ₄ generated at the time of initial charge is small, and the amount of lithium that can be involved in charge / discharge increases. Because it does. In addition, SiO _X having a lithium silicate phase inside has a smaller degree of expansion during charging although the charge amount is the same as SiO _X having no lithium silicate phase inside. Therefore, it is considered that the difference in expansion and contraction during charge / discharge is reduced, and peeling at the negative electrode mixture layer is suppressed.
As the lithium compound used in the heat treatment is not limited to _{LiOH, Li 2 CO 3, LiCl} , or LiF to express same effect it was confirmed. Moreover, it can be estimated that even if it is lithium compounds other than these, the same effect is expressed.

<Second embodiment>
(Example 1)
When LiOH and SiO _X were mixed and heat-treated, a battery was fabricated in the same manner as in Example 1 of the first example except that 2 mol% of LiOH was added to SiO _X. When SiO _X after the heat treatment was analyzed by XRD, a peak of Li ₂ SiO ₃ which is a lithium silicate was confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 0.5 mol%. The battery thus produced is hereinafter referred to as battery B1.

(Example 2)
When LiOH and SiO _X were mixed and heat-treated, a battery was fabricated in the same manner as in Example 1 of the first example except that 50 mol% of LiOH was added to SiO _X. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 12.5 mol%. The battery thus produced is hereinafter referred to as battery B2.

(Example 3)
When LiOH and SiO _X were mixed and heat-treated, a battery was fabricated in the same manner as in Example 1 of the first example except that 80 mol% of LiOH was added to SiO _X. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 20 mol%. The battery thus produced is hereinafter referred to as battery B3.

Example 4
When LiOH and SiO _X were mixed and heat-treated, a battery was fabricated in the same manner as in Example 1 of the first example except that 100 mol% of LiOH was added to SiO _X. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 25 mol%. The battery thus produced is hereinafter referred to as battery B4.

(Experiment)
The batteries B1 to B4 were charged / discharged under the same conditions as those shown in the experiment of the first embodiment, and the initial charge / discharge efficiency shown by the above formula (3) and the 10 shown by the above formula (4) were used. The capacity retention rate at the cycle was examined, and the results are shown in Table 2. Table 2 also shows the results of the batteries A1 and Z.

Batteries A1, B1 to B4 using SiO _X having a lithium silicate phase inside have higher initial charge / discharge efficiency and cycle characteristics than battery Z using SiO _X having no lithium silicate phase inside. Was also found to be good. Moreover, when comparing batteries A1 and B1 to B4, it was found that the higher the ratio of the lithium silicate phase in SiO _X , the higher the initial charge / discharge efficiency and the better the cycle characteristics. Furthermore, in the batteries B2 to B4 in which the ratio of the lithium silicate phase in SiO _X is 12.5 mol% or more, the initial charge / discharge efficiency exceeding the theoretical charge / discharge efficiency (75%) when SiO _X is used as the negative electrode active material. It was confirmed that

From the above, it is desirable that the proportion of the lithium silicate phase in SiO _X is 0.5 mol% or more and 25 mol% or less. When the proportion of the lithium silicate phase in SiO _X is less than 0.5 mol%, the effect of forming the lithium silicate phase is reduced, and when the proportion exceeds 25 mol%, the charge / discharge capacity decreases.

<Third embodiment>
Example 1
As SiO _X (SiO _X before heat treatment) as a raw material, _SiO X (x = 0.93, a carbon coating amount of 10 mass%) Average primary particle diameter of 1.0μm Except for using, the first A battery was fabricated in the same manner as in Example 1 of the example. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 5 mol%. The battery thus produced is hereinafter referred to as battery C1.

(Example 2)
As SiO _X (SiO _X before heat treatment) as a raw material, _SiO X (x = 0.93, a carbon coating amount of 10 mass%) Average primary particle size of 15.0μm Except for using, the first A battery was fabricated in the same manner as in Example 1 of the example. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. Moreover, the ratio of the lithium silicate phase in SiO _X after the heat treatment was 5 mol%. The battery thus produced is hereinafter referred to as battery C2.

(Experiment)
The batteries C1 and C2 were charged / discharged under the same conditions as those shown in the experiment of the first example, and the initial charge / discharge efficiency shown by the above formula (3) and the 10 shown by the above formula (4). The capacity retention rate at the cycle was examined, and the results are shown in Table 3. Table 3 also shows the results of the batteries A1 and Z.

Cell A1, C1, C2 using a SiO _X having an internal lithium silicate phase, than the batteries Z using the SiO _X having no internal lithium silicate phase, high initial charge and discharge efficiency, cycle characteristics Was also found to be good. Therefore, the average primary particle diameter of SiO _X is preferably 1 μm or more and 15 μm or less. In addition, when the average primary particle diameter of SiO _X is less than 1 μm, the particle surface area is large, so that a side reaction of the electrolytic solution easily occurs. On the other hand, when the average primary particle diameter of SiO _X exceeds 15 μm, lithium does not diffuse to the inside of the SiO _X during the chemical conversion treatment, and many lithium silicate phases are formed on the surface of the SiO _X. May cause a drop.

<Fourth embodiment>
Example 1
The SiO _X after heat treatment, washed with pure water until pH of the filtrate reached 8.0, and filtered, except that removal of the lithium compound unreacted from the surface of the SiO _X after the heat treatment, the first A battery was fabricated in the same manner as in Example 1 of the example. The battery thus produced is hereinafter referred to as battery D1.

(Example 2)
A battery was fabricated in the same manner as in Example 1 of the first example except that the following treatment was performed before the heat treatment.
When mixing SiO _X and LiOH, a predetermined amount of SiO _X and a nonionic surfactant (trade name: SN Wet 980, polyether-based surfactant manufactured by San Nopco Co., Ltd.) are added to a solution in which LiOH is previously dissolved in water. Agent) was added and dispersed. In addition, the addition amount of the nonionic surfactant was 1 mass% with respect to the total amount of solid content. Next, the dispersion was dried in a thermostatic bath set at a temperature of 110 ° C., water as a solvent was removed, and heat treatment was performed. The battery thus produced is hereinafter referred to as battery D2.

Example 3
The SiO _X after heat treatment, washed with pure water until pH of the filtrate reached 8.0, and filtered, except that to remove unreacted lithium compound from the surface of the SiO _X after the heat treatment, the fourth embodiment A battery was fabricated in the same manner as in Example 2. The battery thus produced is hereinafter referred to as battery D3.

(Experiment)
The batteries D1 to D3 were charged / discharged under the same conditions as those shown in the experiment of the first embodiment, and the initial charge / discharge efficiency shown by the above formula (3) and the 10 shown by the above formula (4). The capacity retention rate at the cycle was examined, and the results are shown in Table 4. Table 4 also shows the results of battery A1.

  It turns out that the battery D1 which performed the water washing after heat processing improved the first time charge / discharge efficiency and cycling characteristics rather than the battery A1 which did not wash with water. Washing with water as in the battery D1 can remove the lithium compound that is an unreacted substance during the heat treatment, so that the surface resistance of the negative electrode active material particles decreases. Therefore, it is considered that a sufficient conductive path is formed between the negative electrode active material particles during discharge.

In addition, when mixing SiO _X before the heat treatment and the lithium compound, the battery D2, which has been wet-treated using a surfactant in advance, is more than the battery A1 simply dry-mixing the SiO _X and the lithium compound before the heat treatment. It can be seen that the initial charge / discharge efficiency and the cycle characteristics were improved. When a surfactant is added and wet-kneaded as in the battery D1, fine LiOH is uniformly deposited on the SiO _X surface. For this reason, it is considered that a more uniform lithium silicate phase was formed during the heat treatment.

Further, the battery D3 that has been subjected to the wet treatment using the surfactant and the water washing treatment after the chemical conversion treatment has improved initial charge / discharge efficiency and cycle characteristics compared to the batteries D1 and D2 that have been subjected to only one treatment. You can see that Therefore, the characteristics can be further improved by combining the two processes.
Incidentally, from the above experimental results, it was found that preferable to uniformly arrange the LiOH to SiO _X surface, in such a state, not limited to the above wet process, a dry process Can also be achieved.

<Fifth embodiment>
Example 1
[Production of positive electrode]
Lithium cobaltate as a positive electrode active material, acetylene black (HS100, manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder have a mass ratio of 95.0: 2. Weighed and mixed to a ratio of 5: 2.5, and added N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, this was stirred using a mixer (Primix Co., Ltd., TK Hibismix) to prepare a positive electrode slurry. Next, this positive electrode slurry is applied to both surfaces of a positive electrode current collector made of aluminum foil, dried, and then rolled by a rolling roller to produce a positive electrode in which a positive electrode mixture layer is formed on both surfaces of the positive electrode current collector. did. The filling density in the positive electrode mixture layer was 3.60 g / ml.

[Production of negative electrode]
The mixture of SiO _X and graphite after heat treatment used in Example 1 of the first example was used as the negative electrode active material. In addition, the ratio of SiO _X after the heat treatment with respect to the total amount of the negative electrode active material was 5% by mass. The negative electrode active material, carboxymethylcellulose (CMC, manufactured by Daicel Finechem # 1380, degree of etherification: 1.0 to 1.5) as a thickener, and SBR (styrene-butadiene rubber) as a binder Were mixed at a mass ratio of 97.5: 1.0: 1.5, and water as a dilution solvent was added. This was stirred using a mixer (manufactured by Primics, TK Hibismix) to prepare a negative electrode slurry. Next, the said negative electrode slurry was apply | coated uniformly on both surfaces of the negative electrode collector which consists of copper foil so that the mass per 1 m < ² > of a negative mix layer might be set to 190 g. Subsequently, after drying this at 105 degreeC in air | atmosphere, it rolled with the rolling roller, and produced the negative electrode by which the negative mix layer was formed on both surfaces of the negative electrode collector. The filling density in the negative electrode mixture layer was 1.60 g / ml.

[Production of battery]
The positive electrode and the negative electrode were opposed to each other through a separator made of a polyethylene microporous film. Next, the positive electrode tab and the negative electrode tab were attached to the positive electrode and the negative electrode so as to be positioned on the outermost peripheral portion of each electrode, and then the positive electrode, the negative electrode, and the separator were wound in a spiral shape to produce an electrode body. Next, the electrode body was placed in a battery outer package made of an aluminum laminate and vacuum-dried at 105 ° C. for 2 hours. Thereafter, the same non-aqueous electrolyte as the non-aqueous electrolyte shown in Example 1 of the first embodiment is injected into the battery outer package, and the opening of the battery outer package is sealed to make the non-aqueous electrolyte. An electrolyte secondary battery was produced. The design capacity of the nonaqueous electrolyte secondary battery is 800 mAh. The battery thus produced is hereinafter referred to as battery E1.

(Example 2)
A battery was fabricated in the same manner as in Example 1 of the above fifth example, except that in the production of the negative electrode, the ratio of SiO _X after heat treatment to the total amount of the negative electrode active material was 10% by mass. The battery thus produced is hereinafter referred to as battery E2.

(Example 3)
A battery was fabricated in the same manner as in Example 1 of the above fifth example, except that in the production of the negative electrode, the ratio of SiO _X after heat treatment to the total amount of the negative electrode active material was 20% by mass. The battery thus produced is hereinafter referred to as battery E3.

(Comparative Examples 1-3)
As SiO _X, except for using untreated SiO _X (SiO _X non-heat treated), respectively, the battery was fabricated in the same manner as in Example 1 to Example 3 of the fifth embodiment. The batteries thus produced are hereinafter referred to as batteries Y1 to Y3, respectively.

(Experiment)
The batteries E1 to E3 and Y1 to Y3 were charged / discharged under the following conditions, and the initial charge / discharge efficiency and cycle life indicated by the above equation (3) were examined. Table 5 shows the results. The cycle life was defined as the cycle number when the discharge capacity reached 80% of the first cycle. The cycle life of each battery is expressed as an index when the cycle life of the battery Y1 is 100.
Furthermore, the improvement rates in the initial charge / discharge efficiency and the cycle life are those when the batteries having the same mixing ratio of SiO _X are compared. For example, in the case of the battery E1, the improvement rate with respect to the battery Y1. is there.
(Charging / discharging conditions)
After constant current charging at a current of 1.0 It (800 mA) until the battery voltage was 4.2 V, constant voltage charging was performed at a voltage of 4.2 V until the current value was 0.05 It (40 mA). After resting for 10 minutes, constant current discharge was performed until the battery voltage became 2.75 V at a current of 1.0 It (800 mA). [Ratio x / M of the amount M _C of the metal element M contained in the lithium content x and the positive electrode active material for the positive electrode and the negative electrode]
And the amount of lithium x contained in the positive electrode and the negative electrode in these cells, and the amount M _C of the metal element M contained in the positive electrode material, the results were quantified as described above, was calculated x / M _C ratio, Table 5 shows.

As apparent from Table 5 above, it is recognized that the batteries E1 to E3 have improved initial charge / discharge efficiency and cycle characteristics as compared with the batteries Y1 to Y3. Therefore, even in the case of using a negative electrode active material of a mixture of SiO _X and graphite, as SiO _X, it can be seen that it is preferable to use a SiO _X after heat treatment (SiO _X having an internal lithium silicate phase) .
Moreover, it is recognized that the improvement rate in the initial charge / discharge efficiency and the improvement rate in the cycle characteristics are higher as the ratio of SiO _X is higher. However, if the ratio of SiO _X becomes too high, the negative electrode mixture layer may be peeled off significantly. Therefore, the proportion of SiO _X is preferably 20% by mass or less. Incidentally, when the ratio of SiO _X is too small, since the effect of the addition of SiO _X is not sufficiently exhibited, it is preferable ratio of SiO _X is at least 1 mass%.

<Sixth embodiment>
Example 1
[Production of negative electrode]
The mixture of SiO _X and graphite after heat treatment used in Example 1 of the first example was used as the negative electrode active material. In addition, the ratio of SiO _X after the heat treatment with respect to the total amount of the negative electrode active material was 5% by mass. The negative electrode active material, carboxymethylcellulose (CMC, manufactured by Daicel Finechem # 1380, degree of etherification: 1.0 to 1.5) as a thickener, and SBR (styrene-butadiene rubber) as a binder Were mixed at a mass ratio of 97.5: 1.0: 1.5, and water as a dilution solvent was added. This was stirred using a mixer (manufactured by Primics, TK Hibismix) to prepare a negative electrode slurry. Next, the said negative electrode slurry was apply | coated uniformly on both surfaces of the negative electrode collector which consists of copper foil so that the mass per 1 m < ² > of a negative mix layer might be set to 190 g. Subsequently, after drying this at 105 degreeC in air | atmosphere, it rolled with the rolling roller, and produced the negative electrode by which the negative mix layer was formed on both surfaces of the negative electrode collector. The filling density in the negative electrode mixture layer was 1.60 g / ml.

(Preparation of non-aqueous electrolyte)
To a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7, lithium hexafluorophosphate (LiPF ₆ ) was added at 1.0 mol / liter. This was added to prepare a non-aqueous electrolyte.

[Assembling the battery]
In an inert atmosphere, an electrode body was produced using the above negative electrode with a Ni tab attached to the outer periphery, a lithium metal foil, and a polyethylene separator disposed between the negative electrode and the lithium metal foil. This electrode body was put into a battery casing made of an aluminum laminate, and a non-aqueous electrolyte was injected into the battery casing, and then the battery casing was sealed to produce a battery. The battery thus produced is hereinafter referred to as battery F1.

(Example 2)
As SiO _X (SiO _X before heat treatment) as a raw material, _SiO X (x = 0.93, a carbon coating amount of 10 mass%) Average primary particle diameter of 1.0μm Except for using, the sixth A battery was fabricated in the same manner as in Example 1 of the example. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. The battery thus produced is hereinafter referred to as battery F2.

(Example 3)
As SiO _X (SiO _X before heat treatment) as a raw material, _SiO X (x = 0.93, a carbon coating amount of 10 mass%) Average primary particle size of 0.5μm Except for using, the sixth A battery was fabricated in the same manner as in Example 1 of the example. When SiO _X after the heat treatment was analyzed by XRD, peaks of lithium silicates Li ₄ SiO ₄ and Li ₂ SiO ₃ were confirmed. The battery thus produced is hereinafter referred to as battery F3.

(Comparative Example 1)
SiO _X (x = 0.93, average primary particle diameter 15.0 μm) and LiOH 0.2 mol (0.2 mol% of LiOH with respect to SiOx) were mixed using a planetary ball mill, and the average primary particle diameter was 5.0 μm. SiO _X was produced. Further, graphite was added and mixed, and then composited with hard carbon and heat-treated in an Ar atmosphere at 800 ° C. for 5 hours to prepare a negative electrode active material having an average primary particle diameter of 40 μm.
A battery was fabricated in the same manner as in Example 1 of the sixth example except that the negative electrode active material and graphite were 10:90 (SiO: graphite = 5: 95) in mass ratio. The battery thus produced is hereinafter referred to as battery Z1.

(Comparative Example 2)
Using SiO _X (x = 0.93, carbon coverage 10 mass%) with an average primary particle size after ball milling of 1.0 μm, the average primary particle size of the negative active material combined with hard carbon is 8. A battery was fabricated in the same manner as in Comparative Example 1 of the sixth example except that the thickness was 0 μm. The battery thus produced is hereinafter referred to as battery Z2.

(Comparative Example 3)
Using SiO _X (x = 0.93, carbon coating amount 10% by mass) with an average primary particle size after ball milling of 0.5 μm, the average primary particle size of the negative active material combined with hard carbon is 4. A battery was fabricated in the same manner as in Comparative Example 1 of the sixth example except that the thickness was 0 μm. The battery thus produced is hereinafter referred to as battery Z3.
In addition, the negative electrode active material used for the batteries Z1 to Z3 of Comparative Examples 1 to 3 of the sixth embodiment is similar to that of Patent Document 2.

(Experiment)
(Battery performance evaluation)
Since the initial charge capacities of the batteries F1 to F3 and Z1 to Z3 and the initial charge and discharge efficiency shown by the above formula (3) were measured, the results are shown in Table 6. The charging / discharging conditions are the same as the conditions shown in the experiment of the first embodiment.

As is apparent from Table 6 above, it is recognized that the batteries F1 to F3 have improved initial charge capacity and initial charge / discharge efficiency compared to the batteries Z1 to Z3.
The negative electrode active material used in the batteries Z1 to Z3 has a structure in which SiO is dispersed in carbon. On the other hand, the negative electrode active materials in the batteries F1 to F3 have a structure having a thin carbon coating film on the SiO surface. When the particle size of SiO is less than 1.0 μm, it is recognized that the difference in battery characteristics between the structure in which SiO is dispersed in the carbonaceous material and the structure having a thin carbon coating film on the SiO surface is small. On the other hand, when the particle size of SiO is 1.0 μm or more, it can be seen that the structure having a thin carbon coating film on the SiO surface has a larger initial charge capacity and initial charge / discharge efficiency. This is because in the case of the structure in which SiO is dispersed in the carbonaceous material described in Patent Document 2, it is considered that the carbonaceous material covering the SiO serves as a resistance, reducing the utilization rate of SiO during charging and discharging. It is. From the results of Table 6 above, it can be seen that when the structure has a thin carbon coating film on the SiO surface and the particle diameter is 1.0 μm or more, the SiO utilization rate is increased and the initial efficiency is increased.

<Seventh embodiment>
Example 1
2 wt% the proportion of carbon to SiO _X, except that the carbon coverage of the SiO _X surface and 80%, in the same manner as in Example 2 of the first embodiment, a battery was prepared. The battery thus manufactured is hereinafter referred to as battery G1.

(Example 2)
1.5 wt% the proportion of carbon to SiO _X, carbon coverage of SiO _X surface except that was 50%, in the same manner as in Example 2 of the first embodiment, a battery was prepared. The battery thus produced is hereinafter referred to as battery G2.

(Comparative Example 1)
A battery was fabricated in the same manner as in Example 2 of the first example except that the SiO _X surface was not coated with carbon. The battery thus produced is hereinafter referred to as battery R1.

(Comparative Example 2)
A battery was fabricated in the same manner as Comparative Example 1 of the first example except that the SiO _X surface was not coated with carbon. The battery thus manufactured is hereinafter referred to as battery R2.

(Experiment)
The batteries G1 to G2 and R1 to R2 are charged and discharged under the same conditions as those shown in the experiment of the first example, and the initial charge and discharge efficiency shown by the above formula (3) and the above formula (4) The capacity retention rate at the 10th cycle shown in Fig. 7 was examined, and the results are shown in Table 7. Table 7 also shows the results of batteries A2 and Z.

As is clear from Table 7 above, the batteries A2 and G1 to G2 using SiO _X having 50% or more of the surface coated with carbon and having a lithium silicate phase are first charged compared to the batteries R1 to R2 and Z. It can be seen that the discharge efficiency and cycle characteristics are improved.

  The present invention can be applied to a drive power source of a mobile information terminal such as a mobile phone, a notebook personal computer, and a PDA, for example, in applications that require a particularly high capacity. In addition, it can be expected to be used in high output applications that require continuous driving at high temperatures, and in applications where the battery operating environment is severe, such as electric vehicles and power tools.

Claims (7)

Negative electrode active material particles for a non-aqueous electrolyte secondary battery,
The particles comprise carbon only on the surface,
The particles are composed of SiO containing Li silicate phase inside. _X (0.8 ≦ X ≦ 1.2) particles,
SiO _X The surface of the particle is coated with carbon by 50% or more and 100% or less,
Negative electrode active material particles for nonaqueous electrolyte secondary batteries. 2. The negative electrode active material particles for a nonaqueous electrolyte secondary battery according to claim 1, wherein the ratio of the number of moles of the lithium silicate phase to the total mole amount of the SiO _X particles is 0.5 mol% or more and 25 mol% or less. The negative electrode active material particles for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the surface of the SiO _X particles is 100% coated with carbon. 4. The negative active material particle for a non-aqueous electrolyte secondary battery according to claim 1, wherein an average primary particle diameter of the SiO _X particles is 1 μm or more and 15 μm or less. A negative electrode active material for a non-aqueous electrolyte secondary battery, comprising the negative electrode active material particles according to claim 1 and graphite particles . A negative electrode comprising the negative electrode active material particle according to any one of claims 1 to 4 or the negative electrode active material according to claim 5 ,
A positive electrode including a positive electrode active material;
A separator disposed between the positive electrode and the negative electrode;
A non-aqueous electrolyte,
A non-aqueous electrolyte secondary battery. The positive electrode active material includes an oxide containing lithium and a metal element M;
The metal element M includes at least one selected from the group including cobalt and nickel,
The non-aqueous solution according to claim 6, wherein a ratio x / MC between a total amount x of lithium contained in the positive electrode and the negative electrode and an amount MC of the metal element M contained in the oxide is larger than 1.01. Electrolyte secondary battery.